1. Field of the Invention
This invention relates generally to electrodeless discharge lamps, and more particularly to the configuration of the induction coil in an electrodeless discharge lamp.
2. Description of the Prior Art
Electrodeless discharge lamps generally deliver a highly consistent light output over a long life. The most common structure of this kind of lamp includes an induction coil positioned inside or around the outer surface of a discharge vessel. The coil is used to generate an electromagnetic field inside the discharge vessel.
U.S. Pat. No. 4,010,400 describes a lamp device of the kind illustrated in FIG. 1 hereof, wherein a solenoidal or helical induction coil is located in the center of the bulb. As shown in FIG. 1, lamp bulb 1 includes a glass discharge vessel 2 containing a noble gas and a metal vapor. The inner surface of vessel 2 is coated with a fluorescent powder 3, and the vessel is formed around a cylindrical cavity 4 wherein induction coil 5 is placed. A high frequency (HF) generator (not shown) located inside a base 7 generates a high frequency signal to feed induction coil 5.
The operation of the lamp is well known in the art and will be summarized briefly. When the high frequency signal generated by the HF generator is applied to induction coil 5, coil 5 induces an alternating magnetic field H in discharge vessel 2. The alternating magnetic field H in turn induces a secondary electric field E in the discharge vessel 2. FIG. 2 illustrates the general shape of the electrical lines 13 and magnetic lines 12 around the induction coil 5. When the lamp is energized, electrons in the discharge vessel are accelerated by the secondary electric field, forming an electric current 9 (FIG. 1), and the accelerated electrons collide with atoms 10 of the vapor metal. Some of the metal atoms 10 are ionized to maintain the electrical current, but most are excited to a higher energy state. The excited metal atoms fall back from the higher energy state and emit photons 11, predominantly in the ultraviolet range. The ultraviolet photons 11 interact with the fluorescent powder 3 on the bulb wall to generate visible light.
U.S. Patent No. 5,013,975 to Ukegawa discloses an electrodeless discharge lamp in which a substantially helical induction coil surrounds the discharge vessel.
One problem with electrodeless discharge lamps is that their dimming characteristics have generally been very nonlinear. As shown in FIG. 3 (dashed line), the initial decrease in power (region A) causes slight changes in light output that are almost undetectable by the eye; with further reductions in power, the light output drops off rapidly (region B). In short, light output is not a linear function of input power. This means that the efficiency of the lamp varies with input power, and this in turn makes scaling of the lamps difficult. The design of a 12 watt bulb, for example, must be totally different from the design of a 25 watt bulb. A third problem is that electrodeless discharge lamps emit radio frequency interference (RFI) which must be kept to acceptable levels in order not to interfere with radios and other communications equipment.